The long-term goals of this project are to determine the structural (RNA and/or protein) and transmission (replication in Aedes aegypti) characteristics of those dengue type 2 virus variants that have produced dengue hemorrhagic fever (DHF) in humans throughout the world. We have determined the most probable viral structures involved in increased virulence and transmission by studying patient-derived virus variants. Currently we have modified an infectious clone of dengue type 2 virus to contain these structures and developed three assay systems in which to test virus phenotype. We will attempt to correlate dengue virus replication rates in vitro and in vivo with clinical and epidemiologic observations in different geographic areas (U.S. border, Mexico, Peru, Venezuela, Brazil). The identification of these determinants could provide critical information for the design of vaccines and/or anti-virals, which are currently unavailable for dengue. Specific aims: 1. To identify dengue virus factors responsible for increased replication in human and baboon target cells (monocytes and dendritic cells). Hypothesis: Specific viral structures are responsible for increased virus replication rates, which lead to severe disease (DHF) in certain individuals. 2. To identify dengue virus factors responsible for infection and dissemination in the natural mosquito vector from geographic regions with varying dengue epidemiology (presence or absence of DHF). Hypothesis: The mosquito vector promotes the transmission of those virus variants with increased replication rates (viremia) in the human host.